1. Field of the Invention
The invention is directed to a monitoring arrangement for the roll mandrel in a cold pilger mill. The monitor arrangement includes a coil which surrounds the rolled tube concentrically and is arranged on the delivery side after the rolling mill for detecting longitudinal movement of a coil core which is fastened inside the rolled tube to a bar extending between the mandrel and coil coaxially to the tube. Devices for processing the detected movement of the core in the coil are provided to generate a signal by means of which the rolling mill can be stopped.
2. Discussion of the Prior Art
As is well known in the cold pilger rolling art, a metal tube blank is elongated by degrees over a conical roll mandrel and shaped into a tube having a smaller diameter and a smaller wall thickness. In so doing, the roll mandrel is held in position by a mandrel rod. The mandrel rod itself is held in a mandrel rod abutment. The roll mandrel is highly stressed by the rolling force and the friction of the rolling stock and often breaks at different locations. The broken off part then moves along with the tube in the rolling direction. This can damage the rolling mill particularly when the thick part of the roll mandrel wanders into the smaller pass opening of the smoothing rolls which could cause the rolls to spring apart and break.
If the roll mandrel breaks in its thin portion, i.e., in the smoothing pass, the break may not even be noticed in certain cases and the rolled tube or even a number of rolled tubes may have to be scrapped. It may also happen that the roll mandrels themselves are rolled out when the rolling force is too high and high-strength tube material is used. In this case, the roll mandrels are made thinner and the wall of the tube diverges from permissible tolerances. This always leads to a stoppage of the rolling plant and to unacceptable costs.
A roll mandrel monitoring arrangement of the type described above is known, for example, from DE-OS 25 48 379. In this case, a rod carrying a magnetic core at its free end is screwed into the mandrel on the delivery side. If the mandrel breaks, the magnetic core wanders under a coil surrounding the tube and is detected by the coil. This generates an error signal which is used to stop the rolling mill.
A disadvantage of this known arrangement is that the magnetic core must first travel a certain distance under the coil before this coil is activated. Consequently, valuable time is wasted and the machine is sometimes stopped too late to avoid damage and/or significant waste.